The present invention relates to a magnetic bubble generator for a bubble memory in hybrid technology, i.e. having soft or deposited patterns, e.g. of permalloy or FeNi and nonimplanted patterns.
In a magnetic bubble memory, the magnetic bubbles are contained in a monocrystalline magnetic layer, such as a magnetic garnet film, supported by an amagnetic monocrystalline garnet. These bubbles are in the form of generally cylindrical, isolated magnetic domains having a magnetization which is the reverse of that of the remainder of the monocrystalline magnetic layer. These magnetic bubbles are stable by applying a continuous magnetic field H.sub.p perpendicular to the plane of the magnetic layer. In practice, this magnetic field is produced by a permanent magnet, thus ensuring the non-volatility of the informations contained in the bubble memory.
In a magnetic bubble memory, the displacement of the bubbles is brought about by applying a rotary continuous or d.c. field H.sub.T in a direction parallel to the magnetic layer surface. The bubbles are displaced around the so-called propagation patterns.
These patterns are in the form of discs, lozenges, triangles, T's, etc. and can be produced from an iron and nickel-based material, deposited on an insulating layer covering the magnetic layer, or can be obtained by implanting ions in the upper part of the magnetic layer through a mask making it possible to define the shape of said patterns. In the latter case, in view of the fact that ion implantation only takes place around the patterns, the latter are called non-implanted patterns. The propagation patterns are generally contiguous, due to their shape, two adjacent patterns defining between them two cavities.
The displacement of the bubbles along these patterns generally takes place for a time equal to one third of the rotation period of the planar magnetic field H.sub.T, the bubbles remaining stationary in the cavities defined between two adjacent patterns during the remainder of the cycle. These cavities constitute so-called stable positions. Thus, shift registers are produced in which the binary information "1" is represented by the presence of a bubble and the binary information "0" by the absence of a bubble.
Apart from these propagation patterns, it is necessary to use electric conductors for producing in the bubble memory writing, information recording, non-destructive reading, register to register transfer and clearing functions. One of the main types of known magnetic bubble memory comprises a group of loops or registers of the so-called minor type used for storing information, associated with one or two loops or registers of the so-called major type constituting the memory access stations. The minor loops are arranged in a longitudinally juxtaposed manner and the major loops are oriented perpendicular to the minor loops. The magnetic bubbles in the minor loops can be transferred into the major loops and vice versa via unidirectional or bidirectional transfer gates.
When a single major loop is used, the information reading and writing takes place by means of this single loop. In this first case, reference is made to a memory having a major-minor organization. Conversely, when use is made of two major loops, the writing of the information takes place via one of these two loops and the reading of the information via the other loop. These major loops are generally located on either side of minor loops. In this latter case (two loops), reference is generally made to a memory having a series-parallel organization.
A bubble memory comprises three modules, namely a writing module, a storage module and a reading module.
The writing module comprises a bubble generator, a major writing loop and exchange gates for transferring magnetic bubbles from the major writing loop to the minor loops. The storage module comprises a group of minor loops on which are stored the bubbles. The reading module comprises a major reading loop, a group of duplication gates for copying again the magnetic bubbles of the minor loops on the major reading loop, as well as a detection means.
In a magnetic bubble memory in hybrid technology, the writing and reading modules are produced with deposited or soft patterns and the storage module with a non-implanted pattern, so as to permit a maximum information storage density.
According to the prior art, the only patterns used in a magnetic bubble generator in a hybrid technology bubble memory are deposited patterns. FIG. 1 shows an embodiment of a known bubble generator. This bubble generator 2 comprises an electric conductor 5 and a deposited pattern 6. The generally U-shaped electric conductor 5 is deposited on an electrically insulating layer covering the magnetic layer in which travel the bubbles. The deposited pattern 6 is produced on an electrically insulating layer covering the electric conductor 4.
The bubble generation or nucleation position is defined by that part of the internal space of the electric conductor 4 located in the vicinity of one edge of the deposited pattern 6. The latter has a shape serving to produce a strong magnetic pole on said edge.
When a current flows through the electric conductor 4, a magnetic field is produced in its internal space, which locally produces within the magnetic layer a magnetic domain. For a given direction of the electric current passing through the electric conductor 4, the magnetic domain produces a magnetization which is the opposite to that of the magnetic layer, so that said magnetization domain constitutes a magnetic bubble.
The current pulse emitted in the electric conductor 4 must be synchronized with the rotary field H.sub.T. In FIG. 1, the current pulse must be emitted at the time of the phase 1 of the rotary field H.sub.T. Thus, the bubble is produced in position a.sub.1 and then moves with the rotary field H.sub.T into positions a.sub.2 , a.sub.3. At the following time, it crosses the gap between the deposited pattern 6 and a deposited pattern 8 of the major writing loop.
The electric insulating layer separating electric conductor 4 from the magnetic layer containing the bubbles must have an adequate thickness so as not to induce mechanical stresses from conductor 4 on the magnetic layer. However, the greater the thickness of said layer, the more intense must be the nucleation current.
Thus, the choice of the thickness of this electrical insulating layer is a compromise between opposing parameters. Generally a thickness of approximately 100 nm is chosen, which fixes the nucleation current intensity at approximately 200 mA. This thickness is small and can lead to mechanical stress problems.
However, the necessary nucleation current intensity is high for nucleating bubbles at low temperature. However, it is known that the necessary nucleation current increases in proportion to the memory temperature. As the nucleation current is not adjusted as a function of the temperature, but is maintained constant, said nucleation current is generally too high when the memory temperature exceeds approximately 80.degree. C. It is then commonplace for the nucleating magnetic bubble to extend in the internal space along the axis of the electric conductor and splits into two magnetic bubbles when the rotary field H.sub.T passes from phase 1 to phase 2, which produces random errors in the memory.
The magnetic bubble generator shown in FIG. 1 illustrates the conventional structure of a magnetic bubble generator in a memory having deposited patterns or in a hybrid technology memory. In a memory with non-implanted patterns, the structure of the bubble generator is similar to that of the bubble generator of FIG. 1, the only difference being the replacement of the deposited patterns by non-implanted patterns. FIG. 2 illustrates an embodiment of a magnetic bubble generator in a memory with non-implanted patterns.
The major reading loop comprises a sequence of contiguous non-implanted patterns 10, the junction between two successive non-implanted patterns defining a stable position for the magnetic bubbles. A U-shaped electric conductor 4 is arranged perpendicular to the major writing loop axis. The axis of this electric conductor coincides with the junction axis between two consecutive non-implanted patterns. The nucleation position is defined by the base of the internal space of electric conductor 4, said position being a stable position of the propagation path on the major loop.
The magnetic bubble generator of FIG. 2 functions in the same way as the generator of FIG. 1. The nucleation current is emitted into electric conductor 4 in relation with the phase of the rotary field H.sub.T. In the present embodiment, this nucleation current is emitted during phase 3 of the rotary field.
The intensity of the nucleation current is approximately 150 mA, when the electrically insulating layer placed between the magnetic layer and the electric conductor has a thickness of approximately 100 nm.